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https://issues.apache.org/jira/browse/SPARK-25998?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16682017#comment-16682017
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Apache Spark commented on SPARK-25998:
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User 'bkrieger' has created a pull request for this issue:
https://github.com/apache/spark/pull/22995
> TorrentBroadcast holds strong reference to broadcast object
> -----------------------------------------------------------
>
> Key: SPARK-25998
> URL: https://issues.apache.org/jira/browse/SPARK-25998
> Project: Spark
> Issue Type: Improvement
> Components: Spark Core
> Affects Versions: 2.4.0
> Reporter: Brandon Krieger
> Priority: Major
>
> If we do a large number of broadcast joins while holding onto the Dataset
> reference, it will hold onto a large amount of memory for the value of the
> broadcast object. The broadcast object is also held in the MemoryStore, but
> that will clean itself up to prevent its memory usage from going over a
> certain level. In my use case, I don't want to release the reference to the
> Dataset (which would allow the broadcast object to be GCed) because I want to
> be able to unpersist it at some point in the future (when it is no longer
> relevant).
> See the following repro in Spark shell:
> {code:java}
> import org.apache.spark.sql.functions._
> import org.apache.spark.SparkEnv
> val startDf = (1 to 1000000).toDF("num").withColumn("num",
> $"num".cast("string")).cache()
> val leftDf = startDf.withColumn("num", concat($"num", lit("0")))
> val rightDf = startDf.withColumn("num", concat($"num", lit("1")))
> val broadcastJoinedDf = leftDf.join(broadcast(rightDf),
> leftDf.col("num").eqNullSafe(rightDf.col("num")))
> broadcastJoinedDf.count
> // Take a heap dump, see UnsafeHashedRelation with hard references in
> MemoryStore and Dataset
> // Force the MemoryStore to clear itself
> SparkEnv.get.blockManager.stop
> // Trigger GC, then take another Heap Dump. The UnsafeHashedRelation is now
> referenced only by the Dataset.
> {code}
> If we make the TorrentBroadcast hold a weak reference to the broadcast
> object, the second heap dump will show nothing; the UnsafeHashedRelation has
> been GCed.
> Given that the broadcast object can be reloaded from the MemoryStore, it
> seems like it would be alright to use a WeakReference instead.
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